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// Collaborative offboard — move whole VTXOs back to an on-chain output with
// the server's cooperation. Two RPCs: PrepareOffboard (server builds the
// on-chain tx and sends forfeit cosign nonces) and FinishOffboard (we
// countersign a musig forfeit per input; server returns the fully-signed tx
// for us to broadcast). Mirrors bark lib/src/offboard.rs.
//
// The fee math must match the server BIT FOR BIT: captaind recomputes the fee
// and rejects the request unless net_amount == gross - fee exactly
// (server/src/offboards.rs). Rounding mirrors rust-bitcoin 0.32 + bark:
// weight fee rounds up ((rate*wu + 999) / 1000), ppm fees round down.
import { hex } from '@scure/base';
import { sha256 } from '@noble/hashes/sha256';
import { schnorr, secp256k1 } from '@noble/curves/secp256k1';
import * as musig2 from '@scure/btc-signer/musig2';
import { concatBytes, grpcCall, pbWriter, pbFields } from './proto.js';
import { P2A_SCRIPT, pubkeyPolicyTaproot, taprootSighash, txid } from './send.js';
const te = new TextEncoder();
export const P2TR_DUST = 330;
const OFFBOARD_VOUT = 0;
const CONNECTOR_VOUT = 1;
const u32le = (n) => Uint8Array.of(n & 0xff, (n >> 8) & 0xff, (n >> 16) & 0xff, (n >>> 24) & 0xff);
const u64le = (n) => {
const b = new Uint8Array(8);
new DataView(b.buffer).setBigUint64(0, BigInt(n), true);
return b;
};
const varint = (n) => {
if (n < 0xfd) return Uint8Array.of(n);
throw new Error('script longer than 252 bytes not supported');
};
const taggedHash = (tag, ...data) => {
const th = sha256(te.encode(tag));
return sha256(concatBytes(th, th, ...data));
};
// ScriptBuf::new_p2tr(key, None): x-only key tweaked by H_TapTweak(key), no
// script tree — the forfeit tx pays the server this way.
export function bareP2tr(pub33) {
const x = pub33.slice(1);
const tweak = taggedHash('TapTweak', x);
const P = secp256k1.ProjectivePoint;
const out = P.fromHex(concatBytes(Uint8Array.of(0x02), x))
.add(P.BASE.multiply(BigInt('0x' + hex.encode(tweak))));
return concatBytes(hex.decode('5120'), out.toRawBytes(true).slice(1));
}
// Current offboard fee rate in sat/kvB.
export async function getOffboardFeeRate(ark) {
const data = await grpcCall(ark, 'bark_server.ArkService/GetOffboardFeeRate', new Uint8Array(0));
for (const { field, value } of pbFields(data)) if (field === 1) return Number(value);
return 0;
}
// bark: FeeRate::from_sat_per_kwu(sat_vkb / 4)
export const feeRateKwu = (satVkb) => Math.floor(satVkb / 4);
// OffboardFees::calculate — base + ceil(rate * weight) + per-vtxo ppm-by-expiry.
// inputs: [{ amountSat, expiryHeight }]; fees: info.offboardFees.
// The fee splits into what the MINERS take (this offboard's weight in the
// shared tx, at the current rate) and what the ASP charges for the service
// (base fee + the per-coin lifetime ppm, same bracket table as renewals) —
// kept separate so the wallet can show them apart.
export function offboardFeeParts({ spkLen, satVkb, fees, tip, inputs }) {
const wu = (fees.fixedAdditionalVb + spkLen) * 4;
const chainSat = Math.floor((feeRateKwu(satVkb) * wu + 999) / 1000);
// pver >= 4: sub-satoshi ppm accumulates across vtxos, one round-up at the end
let ppmUnits = 0;
for (const v of inputs) {
const blocks = Math.max(0, v.expiryHeight - tip);
const entry = [...fees.ppmExpiryTable].reverse().find((e) => blocks >= e.thresholdBlocks);
if (entry) ppmUnits += v.amountSat * entry.ppm;
}
const serviceSat = fees.baseFeeSat + Math.ceil(ppmUnits / 1_000_000);
return { chainSat, serviceSat, totalSat: chainSat + serviceSat };
}
export function offboardFee(args) {
return offboardFeeParts(args).totalSat;
}
// OffboardRequestAttestation: BIP340 signature over
// sha256(prefix32 || txout(net, spk) || u32le(n) || vtxo ids), per input key.
const OFFBOARD_ATT_PREFIX = te.encode('Ark offboard request challenge '); // 32 bytes
export function offboardAttestation({ netSat, spk, inputIdRaws }, privkey) {
const msg = sha256(concatBytes(
OFFBOARD_ATT_PREFIX,
u64le(netSat), varint(spk.length), spk,
u32le(inputIdRaws.length),
...inputIdRaws,
));
return schnorr.sign(msg, privkey);
}
export async function prepareOffboard(ark, { netSat, spk, rateKwu, inputIdRaws, attestations }) {
const req = pbWriter();
req.varintField(1, netSat);
req.bytesField(2, spk);
req.varintField(3, 1); // deduct_fees_from_gross_amount
req.varintField(5, rateKwu);
const w = pbWriter();
w.bytesField(1, req.finish());
for (const id of inputIdRaws) w.bytesField(2, id);
for (const a of attestations) w.bytesField(3, a);
const data = await grpcCall(ark, 'bark_server.ArkService/PrepareOffboard', w.finish());
let txBytes = null;
const serverNonces = [];
for (const { field, value } of pbFields(data)) {
if (field === 1) txBytes = value;
if (field === 2) serverNonces.push(value);
}
if (!txBytes || serverNonces.length !== inputIdRaws.length) {
throw new Error('malformed PrepareOffboard response');
}
return { txBytes, serverNonces };
}
// The user-side checks from OffboardForfeitContext::validate_offboard_tx.
export function validateOffboardTx(parsed, { netSat, spk, nInputs }) {
const out = parsed.outputs[OFFBOARD_VOUT];
if (!out || out.valueSat !== netSat || hex.encode(out.scriptPubKey) !== hex.encode(spk)) {
throw new Error('offboard tx output does not match request');
}
const conn = parsed.outputs[CONNECTOR_VOUT];
if (!conn || conn.valueSat !== P2TR_DUST * nInputs) {
throw new Error('offboard tx has insufficient connector value');
}
}
// Sign one forfeit per input. Each forfeit spends [vtxo, connector] and pays
// vtxo.amount + dust to the server's bare key. With >1 input the connectors
// come from a deterministic fanout tx that splits the offboard tx's single
// connector output into one dust output per input.
export function signOffboardForfeits({ inputs, keys, serverPub, parsed, serverNonces }) {
const offTxidInternal = hex.decode(parsed.txid).reverse();
const connSpk = parsed.outputs[CONNECTOR_VOUT].scriptPubKey;
const outpoint = (txidInt, vout) => concatBytes(txidInt, u32le(vout));
let connectors;
if (inputs.length === 1) {
connectors = [{ raw: outpoint(offTxidInternal, CONNECTOR_VOUT), txout: parsed.outputs[CONNECTOR_VOUT] }];
} else {
const fanout = {
version: 3, locktime: 0,
inputs: [{ prevout: outpoint(offTxidInternal, CONNECTOR_VOUT), sequence: 0xffffffff }],
outputs: [
...inputs.map(() => ({ valueSat: P2TR_DUST, scriptPubKey: connSpk })),
{ valueSat: 0, scriptPubKey: P2A_SCRIPT },
],
};
const fanTxid = txid(fanout);
// The forfeits spend the fanout's outputs (one dust each) — the sighash
// commits to prevout values, so it must be the per-output dust, not the
// combined connector sum.
connectors = inputs.map((_, i) => ({
raw: outpoint(fanTxid, i),
txout: { valueSat: P2TR_DUST, scriptPubKey: connSpk },
}));
}
const pubNonces = [];
const partials = [];
inputs.forEach((input, i) => {
const k = keys[i];
const tap = pubkeyPolicyTaproot(k.pubkey, serverPub, input.exitDelta);
const forfeitTx = {
version: 3, locktime: 0,
inputs: [
{ prevout: input.point.raw, sequence: 0xffffffff },
{ prevout: connectors[i].raw, sequence: 0xffffffff },
],
outputs: [
{ valueSat: input.amountSat + P2TR_DUST, scriptPubKey: bareP2tr(serverPub) },
{ valueSat: 0, scriptPubKey: P2A_SCRIPT },
],
};
const prevouts = [
{ valueSat: input.amountSat, scriptPubKey: tap.scriptPubKey },
connectors[i].txout,
];
const sighash = taprootSighash(forfeitTx, prevouts);
const nonces = musig2.nonceGen(k.pubkey, k.privkey, undefined, sighash);
const aggNonce = musig2.nonceAggregate([nonces.public, serverNonces[i]]);
const session = new musig2.Session(aggNonce, tap.sortedKeys, sighash, [tap.tapTweak], [true]);
partials.push(session.sign(nonces.secret, k.privkey));
pubNonces.push(nonces.public);
});
return { pubNonces, partials, offboardTxidInternal: offTxidInternal };
}
export async function finishOffboard(ark, { offboardTxidInternal, pubNonces, partials }) {
const w = pbWriter();
w.bytesField(1, offboardTxidInternal);
for (const n of pubNonces) w.bytesField(2, n);
for (const p of partials) w.bytesField(3, p);
const data = await grpcCall(ark, 'bark_server.ArkService/FinishOffboard', w.finish());
for (const { field, value } of pbFields(data)) if (field === 1) return value;
throw new Error('no signed offboard tx in response');
}
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